Density fractionation (usually in sodium polytungstate solution of densities between 1.6 and 2.0 g/cm3) of organic matter allows to gain different pools of OM: free, particulate OM (often called fPOM or light fraction, LF), particulate OM occluded in soil (oPOM, iPOM or occluded light fraction, o-LF) aggregatesand OM associated to mineral surfaces (MOM, MF or H(eavy)F). There are various studies about turnover of these pools, very simplified you can assume the LF is very labile (fast turnover), o-LF is more stable (depending on aggregate stability, intermediate turnover) and HF is accounted to the passive pool (slow turnover). This will give you inside in the situation you have at the moment of sampling. To answer the question if a soil is a sink or a source of OC you always have to measure fluxes. Imagine you will find a soil with high OC content and high amount of OC bound in a passive pool. At the first view, these findings seem to indicate that the soil is a OC sink. Unfortunately, the reality can be totally different: Maybe there was a large active pool which was mineralized during the last century. In this case the total OC content was even higher 100 years ago (but you didn't know) than it is today and the soil was a source of CO2 during this time (and still is today). If you want to get more information on OM fractionation (physical & chemical) check the following review (you will find references for density fractionation there as well): von Lützow et al, Soil Biology & Biochemistry, 2007, 39, 2183-2207

thanks for the answers. Yes i am also thinking of SOC turnover measurements.

to Mr. Markus Graf. Thanks for the suggestions and the article link.